Diverging Diamond Interchange—Going Against Expectations

Imagine driving through a busy interchange and suddenly ending up on the left (or “wrong”) side of the road. Is this a bad dream? A wrong turn? No, it’s a Diverging Diamond Interchange— the latest innovative design in transportation being used to effectively facilitate traffic movement through an interchange.

The Diverging Diamond Interchange was first introduced in 1970s France, but Diverging Diamond Interchange MapAmerica’s first Diverging Diamond Interchange was not built until 2009 in Springfield, MO. Since then, more than 60 Diverging Diamond Interchanges have been built across America, with many more currently in the planning and design stages. As part of the I-4 “Beyond the Ultimate” project team, GAI Consultants worked with the Florida Department of Transportation District Five to deliver line and grade design for a Diverging Diamond Interchange at the Sand Lake Road/I-4 Interchange in Orange County, FL—the first project of its kind in Central Florida.

How it Works

Diverging Diamond Interchanges simplify signal movements by removing all left-turn phasing from traffic signals and converting all turning movements into free-flow movements. This is accomplished by crossing all thru and left-turning traffic to the opposite side of the road through the first simple two-phase signalized intersection. Then, as traffic advances to the opposite side of the road, vehicles that want to make a left turn can proceed in a free-flow movement onto the entrance ramp, and thru traffic continues onto and through the second two-phase signalized intersection, which crosses vehicles back to the right side of the street. This reduced phasing and introduction of free-flow left turns accentuate traffic flow through the interchange with more efficient use of the through and turn lanes.

 

Diverging Diamond Interchange Flow

Challenges Faced

The first hurdle is acceptance of this idea before it has even entered into the design phase. By nature, people are opposed to change, and while the benefits of a Diverging Diamond Interchange design can be easily understood, obtaining approval is not so easy. There is often reluctance to accept anything new until all the kinks have been worked out and the idea is shown to work in the field before it is built in the backyard. But through proper education and outreach, especially during the Project Development and Environment study (PD&E), this idea can be gently introduced to the traveling public and more readily accepted. Once implemented, the biggest challenge that a Diverging Diamond Interchange faces is that it goes against driver expectation, because suddenly the driver is traveling on the left side of the road, or what is considered the “wrong” side of the road in the United States. Proper geometric design, clear pavement markings, and easy-to-understand signage all help with driver acceptance.

The Advantages Are Many

Once the above hurdles are cleared, the benefits of a Diverging Diamond Interchange become apparent, some of which are outlined below:

  • Increased Traffic Efficiency Leads to Reduced Cost: A two-lane Diverging Diamond Interchange can move the same amount of traffic as a three-lane traditional diamond interchange. This efficiency reduces right-of-way acquisition needs and, therefore, results in less overall construction costs.
  • Fewer Signal Controllers: The elimination of left-turn phases simplifies signal timing, which then reduces the number of signal controllers from two to one.
  • Improved Safety: A traditional diamond interchange typically has 24 conflict points—the point at which a highway or roadway user crosses, merges, or diverges with another highway or roadway user—whereas a Diverging Diamond Interchange only has 14. Even more importantly, a Diverging Diamond Interchange reduces the number of more dangerous crossing conflict points from 10 to two, which means reduced risk for accidents.

Is a Diverging Diamond Interchange a “magic bullet” to cure the congestion, operational deficiencies, and safety concerns common to many of the interchanges found along our Interstate system? Not really—this type of interchange tends to degrade operationally if both the through and ramp traffic are heavy; but if ramp traffic is heavy and through traffic is light (or vice versa), it may just be the ticket to a better operating, more efficient, and less costly option.

Jud FohrSenior Technical Manager Jud Fohr, PE specializes in transportation engineering. He has more than 20 years of experience in conventional roadway design, design-build project delivery, and PD&E studies. For questions or additional information about Diverging Diamond Interchange design or the Sand Lake Road/I-4 Interchange project, contact Jud at 407.423.8398.

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